Modern microwave radar and communication systems rely heavily on the performance characteristics of antennas. Continuing advancements in antenna design have resulted in improvements in these systems and have challenged instrumentation manufacturers to provide instruments that can accurately measure the performance characteristics of new antenna designs, such as multibeam antennas, electronically steered phased arrays, dual-polarized and shaped-beam reflectors, and low sidelobe and broadband omnidirectional antennas.
Antenna testing is a multidimensional measurement problem. The received signal from an antenna under test ("AUT") is a complex signal. It is a function of the antenna's frequency, beam position for steered-beam arrays, polarization, spatial position (such as azimuth and elevation angles), and other parameters. The received phasor's amplitude and phase components are principal parameters of operation which are recorded as a function of position and other pertinent test parameters such as frequency and polarization.
In a measurement receiver, it is the principal object to measure the signal amplitude and the phase difference between a signal channel connected to the AUT, and a reference channel connected to a fixed, reference antenna which does not move relative to the transmitter field. Often, measurement values are complex quantities expressed in rectangular form, with an inphase component (I) and a quadrature phase component (Q), which are 90.degree. out of phase with respect to each other.
Reducing the time required to take an individual measurement is governed by a measurement receiver's acquisition speed and AUT positioner speed. Since antenna test positioners have limits on rotational speed, improvements in antenna range productivity are achieved primarily by enhancing receiver performance characteristics. The major receiver characteristics affecting antenna measurements include measurement speed, frequency agility (the ability to change frequencies rapidly while remaining in lock), dynamic range, interface compatibilities with computer-controlled systems, the number of measurement channels, and remote control capability.
Measurement speed is the rate at which a receiver can acquire, process, and output formatted data. The apparent speed of many conventional receivers is usually fast enough that the measurement processing time is transparent to an operator. However, actual receiver speed becomes apparent when repetitive tasks are performed, such as in multifrequency or multiported measurements wherein the frequency can be changed more rapidly than measurements can be processed and provided as outputs.
Fast, coherent, wideband measurements require that a receiver remain phase-locked to the transmit source at all times. A receiver's frequency agility characterizes its ability to maintain phase-lock when encountering rapid frequency transitions. However, particular problems have been encountered in the design of measurement receivers which provide both an acceptable response time and frequency agility.
In such antenna pattern measurement receivers, the magnitude and phase relationships between the signal channel and the reference channel are crucial. In order for this phase/amplitude relationship to be accurately measured, the receiver must be coherent with or locked to a stable time-based reference signal (not to be confused with the signal in the reference channel). In some prior art antenna pattern measurement receivers, the oscillator for the transmitter and the local oscillator for the receiver were phase locked to a common frequency source, and programmed for an intermediate frequency (IF) separation of, for example, 45 MHz. Then, utilizing conventional signal processing techniques, the phase of a 45 MHz IF signal is measured relative to a 45 MHz IF reference channel. In addition, the amplitude is determined by conventional means.
This prior art approach with a common frequency reference source, however, requires a cable or other communication link extending between the AUT's transmitter and the local oscillator of the measurement receiver. Needless to say, it is not always convenient or desirable to require a communication link between transmitter and receiver to maintain oscillator coherency. Moreover, the requirement for coherency limits the use of the measurement receiver to environments wherein such links can be made and precludes testing or measurement of hostile AUTs in the field.
Another difficulty has been encountered in applying new technologies to receivers which utilize this prior art technique. In particular, difficulties have arisen with the use of frequency synthesizers as frequency sources in local oscillator (LO) stages of the receiver. Typical modern frequency synthesizers are digitally controlled and operate in discrete steps of a predetermined frequency interval, for example, 10 kHz, instead of continuous tuning. Many modern RF receivers now utilize both fundamental and harmonic mixing to obtain wideband operation, for example over the RF range of 2-18 GHz, a local oscillator covering 2.0 to 4.6 GHz is used. Unfortunately, if harmonic mixing is to be used with an LO having a digital frequency synthesizer programmable in discrete steps, it may not be possible to select a synthesizer frequency which will harmonically mix with the signal frequency and result in the exact intermediate frequency required.
For example, assume a receiver frequency of 3956 MHz, an LO frequency range of from 1000 MHz to 2200 MHz with a limited tuning resolution of 1.0 MHz and an IF of 45 MHz. Utilizing harmonic mixing it will be seen that the second harmonic of an LO frequency of 2000 MHz is 4000 MHz, which will give an IF of 44 MHz (4000-3956). A 1.0 MHz higher frequency (2001 MHz) gives a second harmonic of 4002 MHz, which results in an IF frequency of 46.0 MHz (4002-3956). The required LO frequency of 2000.5 MHz (0.5.times.4001) cannot be programmed due to the limited resolution of the synthesizer. The IF error in this example is 1.0 MHz, and at higher harmonics the difficulty is further magnified. The actual maximum error encountered is one-half the synthesizer resolution times the mixer operating harmonic number.
Yet another problem encountered in utilizing the conventional prior art approach of a phase locked loop at high RF frequencies with coherent receiver and transmitter sources is frequency switching rate. Systems which utilize an RF phase locked loop typically have limited frequency switching rate capability because of the response characteristics of the phase locked loop. In other words, such prior art systems are generally incapable of switching rates in excess of about 100 frequencies per second. However, modern systems while utilize frequency synthesizers for transmitter sources are capable of frequency switching at rates much higher than this, and it is highly desirable to be able to test systems utilizing such frequency sources.
Accordingly, there is still need for further improvements in radio frequency receivers used in radar, antenna testing, and communications applications, namely, in methods for allowing use of frequency synthesizers as sources for both transmitters and receivers, and noncoherent sources for both transmitter and receiver.